This invention relates to feedback systems wherein nonlinear distortion is reduced to very low levels. It relates particularly but not exclusively to electronic negative-feedback amplifiers for use in sound-reproducing apparatus.
Embodiments of the invention can also reside in integrated-circuit operational amplifiers, in which an outer feedback loop is provided by the user and the inner feedback loops are provided within the integrated circuit. Embodiments of the invention can also reside in instrumentation amplifiers in which extremely low integral and differential nonlinearity are required. Embodiments can also form the basis for repeater amplifiers in telecommunication channels, in which extremely low intermodulation distortion is required. The invention also has application in feedback systems such as used in magnetic amplifiers, hydraulic amplifiers, pneumatic amplifiers, mechanical amplifiers and like signal processing apparatus. Other applications will be apparent to an electronic addressee.
A problem with many types of amplifier is nonlinear distortion. Common specifications include harmonic distortion and intermodulation distortion. A standard method for reducing nonlinear distortion in an amplifier is to apply negative feedback to it.
H. W. Bode (see Bode H. W. "Network Analysis and Feedback Amplifier Design", Van Nostrand, Princeton N.J. 1947) has derived well-known limits to the amount of feedback that can be applied to a single-loop amplifier and some multi-loop amplifiers. Bode's rule for a single-loop low-pass amplifier is that loop gain should not be rolled off faster than about 30 dB/decade if the amplifier is to be unconditionally stable. Conservative practice often restricts the roll-off to 20 dB/decade. Therefore the amount of feedback which could be successfully applied and the consequent reduction in distortion were hitherto limited.